In thin film deposition processes, the substrate, substrate platter, or substrate holder is almost always a flat platen. Due to the flat nature of the platen, non-uniformities in thickness are created in thermal deposition processes. The distance of the platen from the fixed evaporation source is less in the center than at the edge, and more material is deposited per unit time at the center than at the edge. For example, if the center of the source were placed directly under the rotating substrate platen, then a higher rate of deposition would be observed in the middle of the platen, with thinner layers observed on the edges.